Wafer probe

ABSTRACT

The present invention relates to a probe for testing of integrated circuits or other microelectronic devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/997,501, filed Nov. 19, 2001, which application claims the benefit ofU.S. Provisional App. No. 60/251,186, filed Dec. 4, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to a probe for testing of integratedcircuits or other microelectronic devices.

One type of probe utilizes a spaced-apart array of slender needles tocontact pads on a device under test (DUT). A signal is provided to theDUT, and the voltages and/or currents at the selected nodes are routedto measurement equipment. A problem encountered with such measurementsystems, particularly at high frequencies, is that the close proximitybetween the needle tips creates inductance that can interfere withaccurate measurements. Though this inductance can be reduced by limitingthe isolated portion of the probe tips to the region immediatelysurrounding the DUT, practical considerations make such a designdifficult.

Probe structures have been developed to compensate for the inductance atthe probe tips. Orie such design is exemplified by Lockwood et al., U.S.Pat. No. 4,697,143. Lockwood et al. disclose a ground-signal-groundarrangement of strip like conductive traces formed on the underside ofan alumina substrate so as to create coplanar transmission lines. Thesecoplanar transmission lines extend from the pads of the DUT at one endto a coaxial cable at the other end. The associated pair of groundtraces on each coplanar transmission line is connected to the outerconductor of the coaxial cable and the interposed signal trace isconnected to the inner conductor. Areas of wear-resistant conductivematerial are provided to reliably establish an electrical connectionwith the respective pads of the DUT. Layers of ferrite-containingmicrowave absorbing material are mounted about the substrate to absorbspurious microwave energy over a major portion of the length of eachground-signal-ground trace pattern. In accordance with this type ofconstruction, a high frequency impedance (e.g., 50 ohms) can bepresented at the probe tips to the device under test. Thus broadbandsignals of eighteen gigahertz or less can travel with little loss acrossthe coplanar transmission lines formed by each ground-signal-groundtrace pattern.

The probing system of Lockwood et al., however, is insufficient toeffectively probe non-planar surfaces. Such surfaces might result, forexample, if the pads of the DUT differ in height, if a loose metallicparticle of minute dimension adheres electrostatically to the surface ofone of the pads of the DUT so as to form a non-planar surfaceirregularity, or when the plane of the DUT is inadvertently tiltedslightly with respect to the plane of the coplanar tips of the probingassembly. Further, proper alignment between the needles and the DUTrequires careful placement of each needle, a time consuming process.

The alignment limitation between the needles was addressed by Godshalk,U.S. Pat. No. 5,506,515. Godshalk discloses a ground-signal-groundfinger arrangement attached to a coaxial cable, as in Lockwood. Thefingers, however, are originally formed in one piece, joined together bya carrier tab at the contact ends. Once the fingers are attached to thecoaxial cable, the carrier tab is severed and the contact fingersappropriately shaped for contact with the DUT. Godshalk discloses thatthe relative position of each finger is held in alignment first by thecarrying tab, and then by the coaxial cable. Unfortunately, Godshalk'sdesign is limited in that the close placement of a coaxial cable to thefinely spaced geometry of the DUT places a limit on the number ofcoaxial cables, and hence contact fingers, that may be used effectivelyin the probe. Further, a probe having multiple adjacent coaxial cables,each of which has different flexibilities, may lead to insufficientcontact with some of the nodes on the DUT.

Another class of probes that provide clean power to circuits at lowimpedance are generally referred to as power bypass probes. Anotherconfiguration that has been developed to counteract the inductance atthe tips of a probe assembly is a power bypass quadrant. The powerbypass quadrant minimizes such inductance by providing integratedcapacitors or resistor-capacitor networks within the probe.

Strid, U.S. Pat. No. 4,764,723, discloses a power bypass quadrant probethat utilizes an array of ceramic fingers coated with a thin gold orpolyimide film to make contact with the DUT. The test signals are routedthrough a power bypass structure consisting of an RC network. Because ofthe small geometries near the DUT, the capacitors are located far awayfrom the probe tip, which potentially decreases performance. Inaddition, the ceramic contact fingers tend to break during probing,particularly when the probe overshoots the contact pads. Further,probing pads that are not coplanar is exceedingly difficult because theceramic contacting fingers lack flexibility.

Boll et al., U.S. Pat. No. 5,373,231 disclose a probe that includes anarray of blades to contact the pads of a DUT. The array of blades extendfrom a transmission line network traced on a circuit board. An RCnetwork is provided on the circuit board to provide the requisite powerbypass, and in some instances, flexible capacitors are located close to,or between the contact blades. Because of the limited geometries nearthe DUT, the capacitance of the capacitors interconnected between theblades are small, and alone are insufficient to adequately eliminatecircuit inductance. Accordingly, a second bank of capacitors with largervalues are located away from the probe tip where space is available.Probes utilizing flexible capacitors between the closely spaced bladesof the probe have proven to be of limited mechanical durability.

What is desired, therefore, is a configurable, multi-contact probe forhigh frequency testing of integrated circuits or other microelectronicdevices that reduces the inductance at the probe tip to levelsacceptable for measurement over a wide range of frequencies. The probeshould be sufficiently durable and flexible to reliably and repeatedlyprobe substantially non-planar devices over time. It is further desiredthat the probe be easily aligned with the contact points on the deviceto be tested and that the probe be capable of simultaneously probing anumber of such contact points.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a top view of an exemplary embodiment of the probe head ofthe present invention.

FIG. 2 shows a bottom view, at an enlarged scale, of the probe head ofFIG. 1.

FIG. 3 shows an enlarged view of the probe tips attached to a commoncarrying tab of the probe head of FIG. 1.

FIG. 4A shows a schematic of the electrical trace patterns of the topface of the exemplary probe head of FIG. 1 including a power bypassfeature.

FIG. 4B shows a schematic of the electrical trace patterns of the bottomface of the exemplary probe head of FIG. 1 including a power bypassfeature.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIGS. 1 and 2 illustrate an exemplary wafer probe 10. The wafer probe 10includes an integrated tip assembly 12 mounted to a circuit board 14.The integrated tip assembly 12 comprises a plurality of contact fingers16 extending from the circuit board 14 in a radially inward direction soas to match the compact geometry of the device under test (not shown).The distal end portion 17 of each contact finger is shaped to provide areliable electrical connection with an associated pad on a device undertest. The circuit board 14 has electrical traces that route signals fromthe contact fingers 16 through a resistor-capacitor (RC) network 20 topin connectors 22. Measurement cables (not shown) may be electricallyconnected to the pin connectors.

The wafer probe 10 is designed to be mounted on a support through athree hole mounting frame 24 of a wafer probe station so as to be in asuitable position for probing a device under test, such as an individualcomponent on a semiconductor wafer. In this type of application, thewafer is typically supported under vacuum pressure on the upper surfaceof a chuck that is part of the same probing station. Ordinarily an X-Y-Zpositioning mechanism is provided, such as a micrometer knob assembly,to effect movement between the supporting member and the chuck so thatthe tip assembly of the wafer probe can be brought into pressingengagement with the contact pads of the device under test.

Referring to FIG. 3, the integrated tip assembly 12 is fashioned as aunitary device with the individual contact fingers 16 connected by acommon carrying tab 26 at the probing end. Each individual contactfinger 16 is positioned so that, after the integrated tip assembly 12 isattached the circuit board 14, the common carrying tab 26 may besevered, leaving the distal end 18 of each contact finger in theappropriate position for probing the contact pads of the device undertest.

The spacing of the contact fingers 16 at their respective distal ends 18is selected to match the geometry of the DUT pads. Use of an integratedtip assembly 12 advantageously serves to maintain this proper spacingwhile the contact fingers 16 are attached to their respectiveconnections to the circuit board 14. Typically, contact fingers orneedles are attached to a circuit board by being held flush to theirrespective traces and soldered into the appropriate position and pitch.During this process, lateral forces tend to displace the distal ends ofthe contact fingers, making it difficult to maintain the proper spatialrelationship between the contact fingers to match that of the pads ofthe DUT. Use of a carrying tab 26, however, maintains the propertransverse spacing of the distal ends 18 of the contact fingers 16 bycounteracting any lateral forces encountered in the attachment process.

In addition, the probe 16 described herein achieves an improved spatialtransformation between the compact geometry of the microelectronicdevice being probed and the dispersed geometry of the testing equipmentand, if provided, any power bypass circuitry. This improved spatialcharacteristic stands in contrast to earlier design, in which signalswere routed through a coaxial cable. A coaxial cable, having simply aninner and an outer conductor, limits the number of attached contactfingers to three, arranged in a ground-signal-ground arrangement.Accordingly, any common carrying tab used to hold the contact fingers inposition during their attachment to a coaxial cable also is limited to amaximum of three contact fingers.

Oftentimes, however, the DUT has more than three pads to be tested. Insuch a case, configuring the probe requires the use of multiple coaxialcables arranged in an adjacent relationship to each other, usually anawkward process given the limited space available near the probe tips.Use of multiple coaxial cables is also problematical in that differentcables have differing flexibility, making it difficult to line up allthe cables in a single plane and leading to uneven probe forces when thecontact fingers are pressed to their respective pads. Moreover, the usedof multiple coaxial cables and multiple carrying tabs necessitates thecareful and time consuming adjustment of the relative position betweenthe sets of contact fingers to the geometry of the pads of the DUT. Inanother design, the use of multiple coaxial cables and a single carryingtab necessitates the careful and time consuming adjustment of therelative position of the coaxial cables.

Use of a circuit board 14, however, addresses each of these drawbacks.Because the circuit board 14 can include separate traces for each of thecontact pads of the DUT to which the probe will be engaged duringtesting, the common carrying tab 26 depicted in FIG. 3 may include fouror more contact fingers 16, maintaining all of their respective distalends in their proper position until each finger 16 is rigidly attachedto the circuit board 14. The circuit board 14 provides a controlled anduniform flexure, assuring not only a uniform amount of overtravel whenthe fingers 16 make contact with the pads of the DUT, but also amechanism by which the stress in the contact fingers 16 may be relievedby the uniform flexibility of the circuit board 14. This flexibility mayeven be controlled by the selection of material for the circuit board14.

FIG. 3 shows an example of an integrated tip assembly having the commoncarrying tab 26 still attached. The fingers 16 are generally ofrectangular cross section and are preferably composed of the samematerial, where the material is selected from those metals that arecapable of high resiliency to enable the fingers to probe a devicehaving associated contact surfaces that are in non-planar arrangement.In the preferred embodiment, the fingers are formed of beryllium-copper(BeCu) which has been gold plated in order to reduce resistive losses.This material is particularly suited for the probing of contact padsthat are formed of gold, since BeCu is substantially harder than gold.This, in turn, results in minimal wear and a long, maintenance freeperiod of operation of the probe.

If the pads of the device are formed of aluminum instead of gold, it ispreferable to use a harder material for the fingers 16, such astungsten. Here again, the finger material selected is substantiallyharder than the contact pad material in order to ensure minimal wearingof the fingers 16. If tungsten fingers are used, it is preferable thatthey also be gold plated to reduce resistive losses. Use of materialssuch as BeCu and tungsten allows repeated use of the probe whileavoiding the fragility encountered through the use of the ceramiccontact fingers described earlier. It should also be noted that otherpotential materials may be used, in addition to BeCu or tungsten. Inaddition, a number of other potential techniques exist to connect thecontact fingers with the circuit board besides soldering, includingepoxy and the like.

The contact fingers 16 are fabricated as a single, integrated unitattached to a common carrying tab 26 at the distal (tip) ends 18. Thedistal end 18 has a shape that provides a geometrical fanning of thecontacts from the very small pitch (center-to center contact spacing) atthe distal ends 18 up to the larger geometry of the traces 40 on thecircuit board 14.

In accordance with one preferred assembly method, to prepare forconnection of the respective contact fingers to the circuit board,solder paste is evenly applied to the exposed traces on the circuitboard. The fingers are then held just above their corresponding traces,then lowered until they press against the solder paste in an appropriateposition. When the solder is melted, preferably by heating elementsarranged above and below the connection region a solder fillet isdesirably formed between each finger and its corresponding circuit boardtrace.

Preferably, while heating the solder, the fingers are held at a slightdownward incline relative to the distal ends so that during cooling,each finger assumes a planar relationship with the circuit board 14.During this connection process, it will be noted that the propertransverse spacing is maintained between the respective fingers by thecommon carrier tab since any forces that would tend to laterallydisplace the fingers are negated by the common carrier tab 26 that holdsthe contact fingers at their respective distal ends. 18 After thefingers 16 are attached to the circuit board 14, the common carrier tab26 is severed as it is no longer needed because proper finger alignmentis maintained by the circuit board 14. The fingers 16 are preferablyshaped using grinding and lapping processes to create a flat contactarea whose leading edge is visible when viewed from directly above.

Referring to FIG. 1 and FIGS. 4A and 4B, the circuit board electricaltraces 40 provide continued geometrical fanning to even largerdimensions, ultimately leading to one or more connectors such as the setof pins shown in FIG. 1, typically of a much larger physical scale. Thecircuit board 14 may have a ground plane (not shown) providing reducedground inductance and controlled impedance of the signal traces40—usually 50 ohms for use with standard test equipment. Use of acircuit board 14 also allows for the optional use of very smalldimension Surface Mount Technology (SMT) components that can be placedat an intermediate level of geometric scaling.

As shown in FIG. 1 and FIG. 2, the structure is compatible with a powerbypass architecture that can be mounted on the surface of the circuitboard 14. In the preferred embodiment both surfaces of the circuit boardare used to provide the power bypass feature in order to utilize theadditional space.

To illustrate how such a power bypass structure may be incorporated,FIGS. 1 and 2 depict a power bypass architecture spread over bothsurfaces of the circuit board 14. It should be noted, however, that itis entirely feasible to provide a complete power bypass structure usingonly one surface of the circuit board if so desired. In thisillustration, the four contact fingers 50, 52, 54, 56 are arranged in anadjacent relationship, alternating between power and ground contacts. Onthe bottom surface of the circuit board, depicted FIG. 2, a highfrequency metal-insulator-metal (MIM) capacitor is attached between theadjacent power and ground transmission lines formed by respected pairsof contact fingers.

While the MIM capacitor has very low inductive parasitics and a veryhigh self-resonant frequency it does not have very much capacitance.This limits its ability to provide power bypass at lower frequencies.Accordingly, a relatively larger sized and valued SMT capacitor, thoughstill of very small physically dimension, is placed further up the boardwhere there is sufficient space. A small value SMT resistor is used inseries with this capacitor to “de-Q” or spoil the parallel resonancethat can occur between the MIM capacitor and the inductance of the linelength running to the SMT capacitor.

Referring specifically to FIGS. 4A and 4B, the circuit board is designedto allow customization of the function, i.e. ground, signal, power,etc., of each electrical contact of the probe. Initially, each of thefingers is connected to a via to the ground plane, to an SMT componentand eventually to the connector. Programming a ground contact requiressimply leaving the connection to the ground intact, while for all otherfunctions this small circuit board trace is cut away, with a sharp bladeor a laser for instance. When programming a bypassed power line theconnection to the SMT component is left intact while the short circuittrace to the ground is cut.

Referring again to FIG. 1, the probe design preferably includes aninclined circuit board 14 relative to the device under test. A majorportion of the fingers 16 are likewise preferably aligned with the planeof the circuit board 14 with the distal ends 18 being shaped forappropriate probing of the device under test. This inclined designpermits the circuit board 14 to be spaced apart from the device undertest during testing, to increase simultaneously permitting the fingers16 to be short, which minimizes inductance to increase performance.Otherwise, the fingers would need to be mounted in an inclined mannerwith respect to the circuit board, which in many cases, would requirelonger fingers for effective probing.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A probe comprising: (a) a substantially rigid support; (b) a flexiblecircuit board attached to and inclined with respect to said supportbase; (c) an integrated tip assembly comprising a plurality of contactfingers terminating in free space supported by and extending from saidcircuit board such that each of said contact fingers is independentlyflexible with respect to the other said contact fingers, wherein saidcontact fingers are arranged as a unity assembly and interconnected withsaid circuit board while said contact fingers of said assembly beingmaintained in a predetermined alignment.
 2. The probe of claim 1 whereinsaid contact fingers are maintained in a predetermined alignment with atab.
 3. The probe of claim 2 wherein said tab is proximate the ends ofsaid contact fingers.
 4. The probe of claim 3 wherein said tab isproximate the ends of said contact fingers distant from said circuitboard.
 5. The probe of claim 1 wherein said contact fingers extend in aradial direction.
 6. The probe of claim 1 wherein the arrangement ofsaid contact fingers match the geometry of contacting pads on a deviceunder test.
 7. The probe of claim 1 wherein said circuit board includesa respective trace for each of said contact fingers.
 8. The probe ofclaim 1 wherein said contact figures is greater than three.